The invention relates to a viscous coupling having a housing rotatable around a rotational axis and a hub rotatable relative thereto. The housing and hub, together, form an operating chamber. The housing and hub have annular outer plates and annular inner plates which, in an at least a partially radially overlapping way, are alternately arranged in the operating chamber in a certain sequence. The outer plates are non-rotatably connected to the housing and the inner plates are non-rotatably connected to the hub. The outer plates or the inner plates are arranged at a distance from one another in the operating chamber. The coupling has a highly viscous fluid, especially silicone oil, as an operating medium in the operating chamber. The medium, in the case of a relative rotation between the housing and the hub, is subject to shear. The coupling has a compensating chamber which is separated from the operating chamber and which serves to accommodate any fluid which may escape from the operating chamber through a connecting means connecting the two chambers.
DE 26 07 353 A1, published Sep. 9, 1976, describes a viscous coupling. It has a housing having an operating chamber and a compensating chamber. The operating chamber serves to accommodate plates and fluid. In one variant, the compensating chamber is provided in the form of a hub bore. In a further variant, the compensating chamber is provided in the form of at least one bore in the housing. Each compensating chamber has an adjustable piston. Each piston is loaded by a spring. When the fluid expands, the pressure in the housing increases. The pressure causes a displacement of the respective piston(s) from its/their position(s), with a minimum compensating volume. The magnitude of the displacement is determined by springs. In the case of the variant having a hub bore, the fluid enters through slots from the operating chamber into the compensating chamber. Furthermore, the arrangement is such that, in the case of a maximum displacement, the pistons are positioned in an increased diameter bore region. In this piston position, the fluid is able to flow past the pistons and out of the housing.
Such a viscous coupling whose compensating chamber is open towards the operating chamber permits the pressure increase in the housing to be controlled by selecting the spring which loads the piston. Furthermore, from a maximum pressure value onwards, the pressure in the housing is discharged from the housing. Thus torque characteristics are obtained where the torque initially increases in a delayed way, with the pressure remaining constant from a maximum pressure value onwards. From the moment the transmission of torque begins, the pressure increase in the housing is controlled by the piston giving way through compensation of volume as a function of the spring rate. The level up to which the housing is filled with fluid has a considerable influence both on the onset of torque transmission and on the transmitted torque. Loading the pistons by springs initially causes a compensation of volume, with the pressure increasing. This type of embodiment of a viscous coupling causes the torque to be transmitted as a result of the shear effect of the fluid between the plates. As a result, optimum torque transmission is not ensured. When a speed differential occurs, the transmission of torque is additionally delayed.
DE 39 08 090 C1, issued Nov. 11, 1989, describes a further viscous coupling. In addition to the operating chamber occupied by plates, a compensating chamber is provided with a constant volume. When a certain speed limit is exceeded, the two chambers are connected to one another by an aperture which is opened by a slide actuated by centrifugal force so that at high speeds, the coupling is prevented from transferring into the hump mode. When the hump mode occurs, the plates contact one another in a friction-locking way, so that a direct drive occurs. The differential speed has a tendency to approach zero.
As a differential speed sensing system, the viscous coupling has advantages as compared to torque sensing systems as far as traction and driving dynamics are concerned. However, the degressive torque characteristics of the prior art viscous lock can only ever constitute a compromise between traction requirements, permissible driveline torsion, steering reactions and influencing the braking stability.
However, taking advantage of the hump mode has definite advantages in respect of increasing the coupling moment as a result of the thermally conditioned rise in internal pressure when the operating chamber of the coupling is filled completely. This makes it possible to achieve additional traction. However, the disadvantage of utilizing the thermal hump refers to the considerable time span existing between the occurrence of the need for traction and the moment when the operating chamber of the viscous coupling is filled completely and when the pressure increases therein, two conditions to be met for achieving the hump mode.
Differential locks with entirely progressive torque characteristics, admittedly, demonstrate a better traction behavior, but because there is no locking effect at low differential speeds, they do not exert the positive influence on vehicle handling and the driving behavior of the vehicle as a viscous coupling. This is the reason why such torque sensing systems are disadvantageous.